The serotonergic system has close interactions with the dopaminergic system and is strongly implicated in the pathophysiological mechanisms and therapeutic paradigms of Parkinson’s disease (PD). This study aims to investigate regional changes in 5-hydroxytryptamine (5-HT) 2A receptors in the rat brain 3 weeks after unilateral medial forebrain bundle lesion by 6-hydroxydopamine (6-OHDA). 5-HT 2A receptor distributions and alterations in the postmortem rat brain were detected by [3H]ketanserin-binding autoradiography. In the 6-OHDA-induced Parkinson’s rat model, nigrostriatal dopaminergic neuron loss significantly mediated the decreased [3H]ketanserin binding, predominantly in the agranular insular cortex (17.3%, P 5 0.03), cingulate cortex (18.2%, P < 0.001), prefrontal cortex (8%, P 5 0.043), primary somatosensory cortex (17.7%, P 5 0.002), and caudate putamen (14.5%, P 5 0.02) compared to controls while a profound reduction of tyrosine hydroxylase (TH) immunostaining in the striatum was also observed. Alterations in [3H]ketanserin binding in the examined brain areas may represent the specific regions that mediate cognitive dysfunctions via the serotonin system. The downregulation of 5-HT2A receptor binding in this study also provides indirect evidence for plasticity in the serotonergic system in the rat brains. This study contributes to a better understanding of the critical roles of 5-HT2A receptors in treating neurodegenerative disorders and implicates 5-HT2A receptors as a novel therapeutic target in the treatment of PD.